Diana's in the arena on horseback (6)
I believe the answer is:
riding
'on horseback' is the definition.
(I've seen this before)
'diana's in the arena' is the wordplay.
'diana' becomes 'di' (Di is short for Diana).
'in' means one lot of letters goes inside another.
'the arena' becomes 'ring' (I've seen this in other clues).
'di' put into 'ring' is 'RIDING'.
(Other definitions for riding that I've seen before include "Horsemanship - once a third part of Yorkshire" , "Sitting on and controlling the movements of a horse" , "County division for a sporting activity" , "Third Division" , "Establishment teaching horsemanship" .)